1. Field of the Invention
The present invention relates to a rubber composition composed of a block copolymer which is derived from styrene and butadiene as raw materials, and has two kinds of different blocks, the rubber composition being suitable for tire treads.
2. Related Art Statement
There have been recently getting more and more severe requirements for the running stability and the low fuel consumption in the automobiles, and there have been strongly demanded rubbers which are high in wet skid resistance and ice skid resistance and low in rolling friction resistance in addition to the breaking properties such as wear resistance which had been heretofore demanded with respect to the rubber composition for the tread portions of the automobile tires. However, it is difficult to simultaneously meet these properties. Particularly, the wet skid property, the ice skid property and the rolling friction property are in the contradictory relation with one another. Among the conventional polymers, polymers having a relatively high glass transition temperature (Tg) such as an emulsion polymerized SBR are excellent in the wet skid property, while polymers having lower Tg which are represented by a high cis 1,4-BR are excellent in the ice skid property and the rolling friction property. However, the ice skid property is related to the properties at low temperatures, while the rolling friction property relates to the properties at temperatures not lower than room temperature. In order to meet the severe requirements called for regarding the current tires, although the Tg being low is a prerequisite condition so that the ice skid property and the rolling friction property may be excellent, but it is hard to say that the low Tg is a sufficient condition. That is, when the above properties are represented by the viscoelastic indexes of the vulcanized rubber, the wet skid property is represented by tan .delta. near 0.degree. C. (the larger the tan .delta., the more excellent the wet skid property), while the ice skid property is represented by the glass transition temperature (the smaller Tg, the better the ice skid property), and the rolling friction property is represented by the tan .delta. at 50.degree.-70.degree. C. (the smaller the tan .delta., the more excellent rolling friction property). A contradictory question arises when the required properties are simultaneously met. Under these circumstances, there have been made trials for harmonizing the above-mentioned contradictory properties through blending different kinds of polymers together. However, although the properties could be improved to some extent by such harmonization, it is the present situation that all the currently required properties are not met yet.